A conventional amplifier is essentially a variable resistance that uses energy from a direct current (DC) source to increase alternating current (AC) energy. A parametric amplifier uses a nonlinear variable reactance, such as capacitance that varies with time, to supply energy from an AC source to a load. The energy required to vary the capacitance is obtained from an electrical source called a pump. Since reactance does not add thermal noise to a circuit, parametric amplifiers produce less noise than most conventional amplifiers.
Dynamical properties of mechanical resonators can be utilized to create a parametric amplifier. Parametric amplification has long been used as a technique for making a low noise electronic amplifier. The amplification of the applied signal is done by making use of the energy from the pumping action. Parametric amplifiers with a variable-capacitance main-oscillator semiconductor diode are used in radar tracking and communications between Earth stations, satellites, and deep-space stations. The noise temperature of cooled amplifiers is in the range of 20 to 30 K, and the gains are up to 40 dB. This type of amplification is also widely used in optics as well as in electronic traveling wave applications.
Parametric amplification is also observed in micro-mechanical systems. The signal of such a system is obtained by applying the pump voltage at double frequency, making a degenerate amplifier. Similar effects occur in cantilever resonators and torsional resonators. The usefulness of the degenerate amplifier may be limited due to phase sensitivity. It is often undesirable to have an amplifier that requires a specific phase of the signal that is to be amplified.